Once-through boiler control system



Dec. 24, 1968 F, G. SHINSKEY ET Al- 3,417,737

ONCE-THROUGH BOILER CONTROL SYSTEM 2 Sheets-Sheet l Filed Sept.

INVENTOR. FRANCIS G. SHINSKEY JOHN R. LM

ATTORNEY Dec. 24, 1968 F. G. sHlNsKEY ET AL 3,417,737

ONCE-THROUGH BOILER CONTROL SYSTEM 2 Sheets-Sheet 2 Filed Sept. 20, 1.966

ATTORNEY United States Patent O 3,417,737 ONCE-THROUGH BOILER CONTROL SYSTEM Francis G. Shinskey, Foxboro, and John R. Louis, Lexington, Mass., assignors to The Foxboro Company, Foxboro, Mass., a corporation of Massachusetts Filed Sept. 20, 1966, Ser. No. 580,815 7 Claims. (Cl. 122-448) ABSTRACT F THE DISCLOSURE pressure which integration represents a lumped boiler and fuel eiiiciency factor; the power demand signal may be derived from a megawatt demand setting, or alternatively from iirst stage turbine pressure and final steam pressure error; dynamic compensation modiiications are made in the boiler tiring and feedwater control signals to provide for improved dynamic response and for adjustment required by changes in the load level of the boiler.

This invention relates to control systems and more particularly to control systems for regulating oncethrough boilers.

An object of this invention is to provide a control system suitable for regulating a once-through boiler 'incorporating features compensating for boiler dynamics, changes in efliciencies, and fuel heating value.

Another object of this invention is to provide a balance of mass and energy in the boiler-turbine system wherein firing rate is primarily dependent on megawatt demand, and the feedwater flow to tiring rate ratio is dependent on maintenance of rinal `steam temperature.

Another object of this invention is to provide dynamic compensation of a megawatt demand signal to compensate for energy borrowed or absorbed to meet turbine requirements initiated by the change of demand and also to compensate for heat and water storage requirements appropriate to each operating level.

Another object of this invention is to provide dynamic compensation of the tiring rate measurement `supplied to the feedwater control loop, in order to compensate for the difference in the response of tinal steam temperature to changes in feedwater flow yrate as compared with the response to changes in tiring rate.

Another object of this invention is the implementation of an efficiency estimate derived from the integrated difference between megawatt demand and megawatt generation for correcting the megawatt demand signal applied to the firing rate controllers.

Another object of this invention is to derive an etilciency term fro-m the integrated ditference between measured iinal steam temperature and iinal steam temperature set point for trimming the feedwater flow control loop.

Another object of this invention is to derive an efficiency term from the integrated ditference Ibetween final steam pressure and iinal steam pressure set point to rep- `resent changes in the product of the fuel heating value and boiler-turbine efficiency for correcting the demand for thermal power.

Another object of this invention is to provide a feedwater control loop responsive to changes in thermal power demand and corrected for changes in fuel heating rice value and boiler efficiency as well as the required heat pick-up through the boiler.

Another object of this invention is to provide a mass and energy balance control system for a once-through boiler that may be operated either in a boiler-following mode or a turbine-following mode wherein the ltiring rate is made to follow the `selected mode.

Another object of this invention is to provide a oncethrough boiler control system incorporating means for compensating unrequested changes in the heat of combustion of the fuel.

These and other objects and advantages of the invention will be in part apparent from the following detailed description and in part from the drawing hereof, in which:

FIGURE 1 is an embodiment of the invention wherein tiring rate and feedwater liow are dependent on load demand; and

FIGURE 2 is an embodiment of the invention in which tiring rate and feedwater -liow are dependent on the operating level of the turbine, as determined `by throttle valve position and lin-al steam conditions.

The -various analog components illustrated in the FIG. URES 1 and 2, and referred to in the specification below, are stock items and readily obtainable hardware. These analog computational components are covered in detail as to performance and function in the following books: Design Fundamentals of Analog Computer Components by R. M. Howe, Princeton, 1961; Analog Computation 'by Stanley Fifer, Ph. D., New York, 1961, McGraw- Hill.

The boiler control system of the present invention on one hand operates by maintaining an energy balance between ring rate and desired megawatt output, and on the other hand adjusts feedwater flow to maintain desired final steam temperature. These two operations are related by the long term maintenance of the appropriate ratio of feedwater flow to tiring rate.

In steady state operation, megawatt output of the boiler-generator complex is proportional to the effective firing rate to the boiler. Therefore, an energy balance may be maintained by making the tiring rate control primarily dependent upon the demanded megawatt output, so long as relevant conversion etiiciencies and the boiler dynamics are provided for.

Referring to FIGURE 1, the firing rate control portion of the boiler control system of the invention employs as a basic input the megawatt demand set by the operator, or by alternative control means. The megawatt demand signal 111 is restricted between high and low limits as well as to a predetermined rate of change by high-low and rate limiter 11, in order to restrict the demand changes upon the once-through boiler to manageable changes. The modified megawatt demand signal from limiter 11 is applied through dynamic compensator 12 which compensates for energy borrowed or absorbed to meet initial turbine energy requirements when changing to new load levels and also compensates for the heat storage appropriate to different operating levels. Dynamic compensator 12. is effectively a model of the boiler dynamics pertaining to heat storage therein.

The output signal from dynamic compensator 12 thereby includes as a component thereof the transient firing rate required by the boiler dynamics for correcting the heat storage to a new operating level, this component etfectively modifying the steady state tiring rate signal appropriate to the demanded megawatt output.

The modified megawatt demand signal from rate and level limiter 11 is also compared with the measured megawatt output 13 of the generator by comparator 14. The output of comparator 14- is the deviation between megawatt demand and generated megawatts. This deviation is applied to proportional-plus-reset controller 1S, the long-term integrated output being a signal representing changes in the efficiency of the boiler-turbine complex and changes in fuel heating value which are assumed to be responsible for deviation between demand and generation.

The megawatt demand signal from dynamic compensator 12 is applied to a first input of multiplier 16. The integrated output from proportional plus reset controller 15 which represents the reciprocal of the product of fuel heating value multiplied by the total efficiency of the boiler-turbine complex, is` applied to the second input of multiplier 16. The -output of multiplier 16 represents the actual required firing rate Q necessary to maintain the proper balance between firing rate and megawatt output. Firing rate Q equals:

H 0 Eff boiler-turbine where,

Q is actual firing rate MW is megawatt demand Hc is fuel heating value E boiler-turbine is the product of the efficiencies of the boiler-turbine complex The output of multiplier 16 is applied to the input of dynamic compensator 17, and also through summer 18 to the input of dynamic compensator 19'. The two compensators 17` and 19 work in conjunction, compensator 17 to dynamically characterize the input to the tiring rate portion of the control system, while compensator 19 dynamically characterizes the input to the feedwater portion of the control system, compensator 17 and 19" thereby minimizing temperature transients following changes in megawatt demand.

The output of Adynamic compensator 17 is applied to a first input of multiplier 20, the output of which provides the set point input to proportional-plus-reset controller 21. Controller 21 has applied t-o its measurement input a signal derived from a measurement of the firing rate 23, such as from a ow signal for 'a fuel oil firing, or from a tachometer signal on a pulverized coal feeder. The output of controller 21 regulates the fuel-air controllers 24 -so that the measured firing rate is made equal to the set point of controller 21.

The fuel and air controllers operate the pulverized coal feeders, for example, in parallel with the combustion air blowers; the coal feeders are limited so as always to guarantee an excess of air supply.

Comparator 22 and summer 18 function to modify the feedwater flow set p-oint supplied to the input of dynamic compensator 19 should measured firing rate not equal the demanded firing rate.

The measured firing rate 23 is differenced by comparator 22 with the firing rate set point signal at the output of multiplier 20. The output of comparator 22 is the measured error in firing rate, and this output is supplied to summer 18 to be added to the signal Q (required firing rate) from the output of multiplier 16, referred to above. The sum of the error and required tiring rate Q is the actual firing rate. This sum is supplied to the input of dynamic compensator 19, which transfers this sum to provide a set point for the feedwater portion of the control system. Thereby, a coal feeder failure, for example, will cause the feedwater flow to be reduced commensurately with the reduction in measured firing rate 23; thus steam temperature error is reduced.

In -addition to providing a basis for the overall boilerturbine efficiency term for the firing rate, the output of comparator 14 is also employed to provide dynamic corerection for the turbine throttle pressure set point, which is thereby varied to permit more Irapid response of the turbine to demand changes. The output of comparator 14, representing deviation between megawatt demand and megawatt generation, is -applied to one input of comparator 25. The final steam pressure set point 26 is applied to the other input of comparator 25, and the output of comparator 25, being a modified final steam pressure set point, is applied to set point input 27 of a conventional proportional-plus-reset controller 28. The measured final steam pressure 29 is applied to measurement input 30 of controller 28. Under steady-state conditions of operation, the input to comparator 25 from comparator 14 is zero as megawatt demand will equ-al -megawatt generation, and the turbine governors 31 are controlled only by controller 28 on the basis of deviation between final steam pressure set point 26 and the nal steam pressure measurement 29.

If the demand or load changes a megawatt demandgeneration deviation momentarily occurs, and the comparator 14 output applied to the input of comparator 25 will be subtracted from iin-al steam pressure set point 2-6. Thereby, controller 28 receives a modified `set point from comparator 25 and the output ofy controller 28 immediately regulates the turbine governors 31 and `changes the throttle pressure in a direction which tends to reduce megawatt deviation. This set point 27 modification tends to maintain the proper generator output over the time the boiler-generator combination requires to adapt to the new firing rate appropriate to the new demand or load level. When megawatt generation eventually equals megawatt demand, the comparator 14 output becomes zero, and the controller 28 regulation of the turbine pressure will again depend solely on regulating the measurement of final steam pressure 29 to set point 26. 1n this manner, the turbine responds more quickly to demand or load changes.

The feedwater flow portion of the boiler control system employs a mass and energy balance method based on the required final steam temperature. The rate of heat input to the boiler determines the feedwater fiow required in order to maintain the desired final steam temperature. The long-term rate of feedwater flow required is proportional to the firing rate, and inversely proportional to the change in enthalpy of the feedwater passing through the boiler. The feedwater control portion of the system employs final steam temperature to maintain over the long term the proper ratio of feedwater flow rate to firing rate.

Heat absorption of the boiler is deter-mined by comparing entering and exiting enthalpy of the feedwater in the form of steam as shown:

boiler heat absorption feedwater ow=exit enthalpyentering enthalpy Heat absorption of the boiler is proportional to firing rate and is also affected by the heating value of the fuel and the boiler efficiency, or

boiler heat absorption: Q Hc E boiler substituting for the term heat absorption,

Q is the firing rate Hc is the heating value of the fuel Eff boiler is the boiler efficiency HS is the exit enthalpy Ht is the entering enthalpy The long-term feedwater fiow rate is responsive to the compilation represented by this last formula.

The final steam temperature 32 is measured and compared with the desired final steam temperature set point 33 by comparator 34. The output of comparator 34 is vthe final steam temperature error which is integrated by proportional plus reset controller 35 and applied to the numerator input 36 of divider 37. The integrated temperature difference from controller 35 compensates for changes in the heating value of the fuel and changes in the `boiler efficiency. Thus input 36 represents:

H c Eff boiler Variations in Ifeedwater enthalpy are determined by suit-ably scaling the entering feedwater temperature obtained by sensor 38 to represent feedwater enthalpy Ht. Ht is subtracted from a fixed factor 39 representing de sired final steam enthalpy Hs by comparator 40. The output of comparator 40, HS-Ht, represents the desired enthalpy change through the boiler. The output of comparator 40, Hs-Ht, is applied to the denominator input 41 of divider 37. The output of divider 37 thereby represents the term:

QXHcXEff bOllel Hs-Ht This term is applied vfrom the multiplier 42 output to set point input 44 of conventional proportional-plus-reset controller 45. Controller 45 operates the boiler-feed pump governor 54. The measurement input 46 of controller 45 is derived from a fiowmeter 47 at the outlet 48 of boiler feed pump 49. Flowmeter 47 monitors the boiler feed pump output and may be a fiow nozzle in the fiow line 50 monitored by a differential pressure measuring device 51, the output of which is applied to multiplier 52. The other input of multiplier 52 is derived lfrom 4boiler feedwater temperature sensor 38 through function generator 53 in order to temperature-compensate the ow rate signal. Function generator 53 is necessary to convert the temperature signal from sensor 38 into density which converted signal is applied to multiplier 52. The output of multiplier 52 is square-rooted by square root extractor 58, the output of which is the square root of the product of differential pressure and density, thus representing feedwater mass fiow. This output of square root extractor 58 is thus a temperature-compensated mass fiow signal which is applied to measurement input 46 of controller 45. The controller 45 `output to pump governor 54 thereby regulates boiler feed pump 49 and the feedwater mass fiow rate through line 50 in accordance with the set point supplied to set point input 44 of controller 45.

The temperature difference between nal steam temperature and set point, in addition to being supplied t0 controller 35, is .also taken from the output of comparator 34 and supplied to proportional-plus-derivative controller 55 f-or regulating the spray Water control 56. By these means, transient deviations of final steam temperature may be immediately employed to adjust the spray water to trim the final steam temperature 32 to the desired set point 33. The spray water inlet 57 may be connected to boiler feedwater line 50 on the boiler feed pump 49 side of fiowmeter 47, or on the downstream side of flowmeter 47. Thus, spraywater fiow may or may not be included in the measurement of feedwater flow by flowmeter 47 as desired.

The delay in the response of megawatt output to firing rate changes, and the delay in the response of steam temperature to feedwater flow changes, vary inversely with load. Ordinarily, this delay variation results in feedback control loops which are marginally stable at low loads and overdamped at high loads. In the control system configuration of this invention, however, multiplier 16, multiplier 20 and multiplier 42 are located in the above feedback loops effectively changing their gain proportionally to the load, and thus changing control loop gain in order to compensate for the variations in process characteristics.

Compensation for unrequested. variations in effective firing rate, such as variations in the fuel heating value, is provided by a feedback connection from final steam temperature deviation to the firing rate controller.

The final steam temperature error signal available at the output of comparator 34 is coupled through proportional controlier 59 to the second input of multiplier 20, referred to above. This feedback signal, acting through multiplier 20, effectively modifies the ratio of firing rate to power demand in order to compensate for sudden changes in heat of combustion or boiler etiiciency that tend to produce deviations in final steam temperature, as well as in megawatt generation. The feedback signal is applied on a load adaptive basis, compensating for changes in process dynamics with load. The effect of the feedback signal applied to multiplier 2t) is qualified by the level of demand signal applied to the other input of multiplier 20.

FIGURE 2 depicts an alternate embodiment of the invention in which the firing rate is derived from first stage turbine pressure and final steam pressure error in which final steam pressure error is an indication of the difference between thermal megawatt demand and generation, and may be employed during conditions in which the speed changer motor (or turbine speed set point) is at a fixed position. The load demand then is proportional to the setting of the turbine governor as established by the speed changer motor. In FIGURE 2 the speed changer motor is shown as vload demand 60 which is the set point for the turbine governor control 31.

When the turbine throttle valve is fixed, the first stage turbine pressure measurement 61 is a measure of thermal power. Final steam pressure measurement 29, which is practically the same as the turbine input pressure, will be affected by changes in operating conditions of the boiler. To develop an accurate feedforward signal controlling firing rate, the thermal power fiow determined by the first stage pressure measurement 61 must be modified by the final steam pressure 29 deviation from set point 26, thereby providing a signal representative of thermal power demand. The subtraction of final steam pressure 29 from set point 26 by comparator 63 produces the final steam pressure error which is added to first stage pressure 61 by summer 62. The output of summer 62 is a feedforward firing rate demand signal compensated for transient variations in final steam pressure.

The output of summer 62 is supplied to the input of dynamic compensator 12 modeling the characteristics of the boiler so as to compensate for the first stage turbine pressure response, that is to say steam flow response, to changes in firing rate.

Steady-state errors in final steam pressure 29 are a result of changing boiler efficiency or fuel heating value and may be compensated by integrating the first stage pressure error at the output of comparator 63 by proportionalplus-reset controller 15 and combining the integrated term with the firing rate signal from dynamic compensator 12 by means of multiplier 16, the firing rate signal being applied to a first input and the integrated efiiciency term to the second input of multiplier 16. The output of multiplier 16 is the error-corrected firing rate control signal.

7 The remainder of the control system of FIGURE 2 functions in the same manner as the duplicate like-numbered portion of the control system of FIGURE 1.

While there has been shown what is considered to be a preferred embodiment of the invention, it will be manifest that many changes and modifications may be made therein without departing from the essential spirit of the invention, It is intended, therefore, in the annexed claims to cover all such changes and modifications as fall within the true scope ofthe invention.

What is claimed is:

1. A once-through boiler control system for producing a firing rate control signal and a feedwater tiow control signal comprising:

a firing rate control computation section including comparison means to determine the difference between a signal corresponding tot nal steam pressure and a signal corresponding to final steam pressure set point and including integrating means responsive to the output of said comparison means with said integrating mea-ns having an output corresponding to an'integraton of said difference and signal modifying means having a first input responsive to said output of `said integrating means and having a second input responsive to a power demand signal wherein said power demand signal appearing at said second input is modified by said output of said integrating means appearing at said first input with said modifying means having an output corresponding to the appropriate firing rate over the long-term, and

a feedwater flow control computation section having a first input derived from power demand and having a second modifying input derived from an integration of the difference between final steam temperature and final steam temperature set point whereby the output of said 4feedwater flow control computation section corresponds over the long term to the requisite feedwater flow for maintaining final steam 'temperature at the desired set point.

2. The once-through boiler control system of claim 1 wherein Isaid power demand signal is lderived from rst stage turbine pressure and final steam pressure error.

3. The once-through boiler control system of claim 1 with means lfor modifying the firing rate in accordance with final steam temperature error thereby compensating for changes in the heat of combustion.

4. A once-through boiler control system for producing a tiring rate control signal and a feedwater ow control signal comprising:

a firing rate control computation section including comparison means to determine the difference between a signal corresponding to power demand and a signal corresponding to power generation and including integrating mea-ns responsive to the output of said comparison means with said integrating means having an output corresponding to an integration of said difference and signal modifying means having a first input responsive to said output o-f said integrating means and having a second input responsive to said power demand signal wherein said power demand signal appearing at lsaid second input is modified by said output of said integrating means appearing at said first input with said modifying means having an output corresponding to the tiring rate appropriate to said power `demand over the long term, and a feedwater flow control computation section having a first input derived from power demand and having a second modifying input derived from the integration of the difference between the final steam temperature and final steam temperature set point whereby the output of said feedwater ow control computation section corresponds over the long term to the requisite feedwater flow for maintaining final steam temperature at the desired set point.

5. The once-through boiler control system of claim 4 wherein the product of said first and second inputs to said feedwater flow computation control section is divided by the difference between boiler feedwater enthalpy and desired final steam enthalpy to produce the feedwater flow rate control signal.

6. The once-through boiler control system of claim 4 wherein said first input to said feedwater ow control computation section is modified by any deviation in measured firing rate from firing rate set point in order to maintain proper feedwater flow in the event said measured firing rate deviates from the firing rate appriopriate to said power demand.

7. The once-through boiler control system of claim 4 wherein said modifying means performs a multiplicative function so that said power demand is multiplied by a coefficient corresponding to power output error.

References Cited UNITED STATES PATENTS 3,109,102 10/1963 Jenkins 296-2 3,244,898 4/1966 Hickox 290-2 3,286,466 1l/1966 Stevens 60-105 3,310,683 3/ 1967 Hottenstine 290--2 CARROLL B. DORITY, Primary Examiner. 

